Let’s Talk About Creepage & Clearance

I’m mostly a functional safety guy but I’m also very interested in cyber security, intrinsic safety and data protection. Electrical safety is one I don’t really claim to know much about but on the other hand as a safety person you can’t ignore it either as there as overlaps with some of the above areas.

One aspect of electric safety that is very important for someone doing intrinsic safety designs at the integrated circuit level is creepage and clearance. Creepage and clearance are also important for functional safety where the numbers used for creepage, and clearance can also be used to justify separations used on PCBs where independence is claimed.

Creepage & Clearance Explained

First to explain the terms, creepage is the shortage distance along an insulating surface or surfaces between two conductors. The two conductors could be the pins on an IC (integrated circuit) or tracks on a PCB. In both cases think of a liquid creeping along the surface.

Clearance is similar but is the distance in air between the two points. The graphic below shows one example. Clearance is often shown as a rope between two points pulled tight vs creepage which is the rope going slack until it supported by a surface.

Figure 1 – Creepage and clearance

Creepage is always/generally larger than clearance unless special measures are taken. This might include putting a notch in a PCB so that the creepage distance is extended by twice the depth of the notch. The clearance (measurement of distance in air) will be unaffected but the creepage (distance along the surface) is increased.

It won’t come as a shock that how sensitive a system is to creepage depends on the material used and the likelihood of contamination. Liquids creep along the surface of some materials better than others. The relevant property of the material is its CTI (comparative tracking index) and is generally expressed as a material group as shown below. 

Figure 2 – Comparative tracking index

CTI & Pollution Degrees

CTI is measured according to IEC 60112 by applying a 0.1% ammonium chloride solution to the material and a voltage is applied across the insulator, allowing a current to flow. The material is rated for the highest voltage for which carbon tracks are not formed on the surface.

A higher value of CTI is better as it means more resistance to forming a conducting path across the material. If you don’t know any better, IEC 60112 requires that you assume the material is material group IIIb (i.e. the worst/most conservative assumption).

According to the ADI technical article MS-2203 entitled “The relation between the comparative tracking index and working voltage” the CTI of the packages used for ADI’s digital isolators has been measured as > 600V which means material group I but conservatively ADI use material group II in their claims. I presume all semiconductor packages are similar as I don’t think ADI’s digital isolators do anything special.

Of course, if you keep out the liquids then the CTI doesn’t really matter. Pollution degree is a measure on a scale of 1 to 4 on how likely there is something there to form the conductive path.

Figure 3 – Pollution degree classification

For electric safety the working voltage, material group and pollution degree combine to give minimum creepage requirements (remember clearance – the distance through air - doesn’t depend on surface properties or contamination).

Figure 4 – Basic insulation creepage requirements in mm from IEC 60950-1-1.

So, for a typical industrial 24V system with a high IP rating to keep out pollution the creepage requirement for a digital isolator would be only 0.18mm.  If however, you are deemed pollution degree 3 then for an IC with a surface rated as material group II your creepage requirement has increased from 0.18mm to 1.7mm if you want to rule out shorts.

• Note: IP (ingress protection) comes from IEC 60529 and rates and grades the resistance of enclosures against the intrusion of liquids and dusts. An example would be an IP54 enclosure.
• Note: If your working voltage is not shown above, I believe you must use the next highest voltage in the chart rather than interpolating but I would have to get out my standard to double check.

An example of where creepage and clearance are used for functional safety is found in ISO 13849-1:2015.  In IEC 61508 I only see in mentioned in part 7 but would have expected to see it in part 6 annex D where it discusses “A methodology for quantifying the effect of hardware-related common cause failures in E/E/PE systems”. Perhaps IEC 61508 needs a bit more on the topic of PCB design!

Figure 5 – extract from ISO 13849-1:2015 table F.1 entitled “Scoring process and quantification of measures against CCF”.

If you ever wondered when you read ISO 13849-1 (a machine control system safety standard) what was meant by sufficient clearance and creepage perhaps this blog will help you.

For Analog Devices this means that sometimes our digital isolators come in what may look like very old, very big packages. However, this is not the case as even state of the art digital isolators with the highest speeds, reliability and isolation quality are “forced” to use these packages to meet the isolation requirements when there might be 1000 or more vols present such as for motor control applications. So, for instance, the ADuM320N comes in an 8-lead wide body SOIC package with a body width of 7.5mm and the ADuM1440 is available in a QSOP with a body width of 4mm.

Figure 6 – package dimensions for the ADuM320N with increased creepage

The intrinsic safety standards are very conservative and not generally based on probability. The goal is to make it impossible for an explosion to occur even if a combustible gas is continuously present. IEC 60079-11:2023 gives the clearance and creepage requirements when making no assumptions on the housing (lower values can be used if the housing is IP54 or better).

Figure 7 -  An extract from intrinsic safety standard IEC 60079-11:2023 showing required creepage and clearance distances

If the actual creepage and clearance values exceed that in table 7 you are done, you get a fault exclusion (the intrinsic people refer to it as infallibility) on a short between the two connections or circuits. If it’s between the value in the table and one third of that value, it is considered a countable fault (I will blog on this another day) but for now let’s just say you need to analyze the circuit for at least two countable faults when trying to achieve the highest level of Intrinsic safety Ex ia. If the actual creepage and clearance values are less than 1/3 of the values in the table, you should assume the two circuits or connections are permanently shorted. Given the lack of guidance in functional safety standards on this topic I think the information from the intrinsic safety standard is very useful for CCF (common cause failure analysis).

• Note: I based some of the material above from a Maxim Integrated (now part of ADI) article entitled “Understanding the certification of digital isolators”. The article is now out of date since IEC 60747-17 has existed since 2020 as a specific standard for digital isolators. IEC 60747-17 is now recognized in other standards such as the intrinsic safety standard IEC 60079:2023 and hopefully will be recognized in more standards as time goes by.

About the Safety Matters Blog Series

For the relevance of this blog to intrinsic safety please see the Analog Devices technical article entitled “Digital isolators can be used in intrinsic safety applications”.

For previous blogs in this series see here.

For the full suite of ADI blogs on the EngineerZone platform see here.

For the full range of ADI products see here.

The next blog in this series will be on the second Tuesday of next month but in the meantime please follow me on LinkedIn for weekly short posts.